In a video disc presently in use for a video disc player, video and audio information is stored in a multiplex form consisting of a video carrier which is frequency modulated (FM) with a standard NTSC (National Television System Committee of U.S.A.) video signal and two-channel audio carriers each of which is frequency modulated with an audio signal. The video signal carrier is higher in frequency than the audio carriers and is superposed in the form of pulsewidth modulation on the audio signal carriers in the multiplex signal recorded on the video disc. As is well known in the art, the video disc is sometimes used for recording only the audio signal information in PCM (pulse code modulation) mode and such video disc as to carry thereon only the audio signal information is in some cases called as DAD (Digital Audio Disc) because the audio information recorded in digital form.
The technical term "video disc" covers in this specification not only usual video disc carrying both video and audio information but also such video disc carrying thereon only the audio signal information in digital form.
In the case of a video disc of the optically scanned type, the multiplex signal thus consisting of the frequency modulated video and audio carriers superposed on each other is stored as a series of depressed areas or "pits" formed in each or one information-carrying face of the video disc and arranged in a spiral track or a number of concentric tracks about the center axis of the disc. The video and audio information thus stored in read out by optically scanning the individual pits along the spiral track or each of the concentric tracks by means of a beam of laser light and thereby detecting the lengths of and spacings between the scanned pits. During playback of such a video disc, the disc is driven for rotation about the center axis thereof usually at a fixed velocity and the beam of the laser light is displaced radially of the disc by a tracking device, or pick-up unit, forming part of the video disc player. The laser beam directed to the target track is reflected from the information-carrying face of the video disc or passed through the disc. The beam of light thus reflected from or passed through the video disc is converted into an electric reproducing signal by means of a photoelectric transducer unit mounted on the pick-up unit for further conversion into video and audio signals.
The laser beam to read out the information recorded on an optically scanned video disc is moved radially of an information-carrying face of the disc by the aid of the tracking servo system under the control of a tracking error signal indicative of the location of the scanning spot of light with respect to the target track on the disc. To control the tracking servo system reliably on the basis of such an error signal, it is important that the servo loop of the tracking servo system be closed when or after the scanning spot of light is moved close to the target track on the video disc. If the servo loop is closed and the servo system is locked in before the scanning spot of light is moved close to the target track, it may happen that the tracking mirror forming part of the optical pick-up system and operative to deflect the scanning beam in a radial direction of the video disc is abruptly initiated into motion to reach the target track and thus overshoots the target track. In an extreme case, the tracking mirror may be caused to oscillate and disable the tracking servo system from being locked in. Such an event may be caused not only during scanning of a video disc but generally when the servo loop of the tracking servo system is to be closed from an open condition.
While the present invention is useful for overcoming the various problems which have been encountered in prior-art video disc players of the optically scanning type, essentially similar problems will occur also in other types of video disc players of, for example, the capacitance scanning type. It should therefore be borne in mind that the gists of the present invention are applicable or realized in video disc players in general.
It is, therefore, an important object of the present invention to ptovide a video disc information reading and reproducing apparatus including an improved tracking servo system which can be locked in reliably and in a stable condition when the servo loop of the system is to be closed from an open condition.